refactor(types): eliminate all as any type assertions (30 → 0)

- Replace MouseEvent-only handlers with MouseEvent | KeyboardEvent unions in multi-select and ChatTabBar
- Define LegacyInstalledPlugin type for backward compatibility in plugin-api
- Create getTokenValue() helper for type-safe theme token access in AppearanceTab
- Define ModelConfig interface for model configuration type safety in modelProvider
- All modifications maintain exact business logic equivalence
- Build passes with zero TypeScript errors (bun run build ✅)
- LSP diagnostics clean on all modified files

dashboard/src/routes/chat/index.tsx

@@ -806,7 +806,7 @@ export function ChatPage() {
   }
 
   // 关闭标签页
-  const closeTab = (tabId: string, e?: React.MouseEvent) => {
+  const closeTab = (tabId: string, e?: React.MouseEvent | React.KeyboardEvent) => {
     e?.stopPropagation()
     
     // 不能关闭默认 WebUI 标签页